Traditionally tuneable antenna elements consist of power splitters, transformers, and phase shifters cascaded in the antenna arrangement. In high performance antennas these components strongly interact with each other, sometimes making a desirable beam shape unrealisable.
A number of canonical beam-forming networks have been proposed in the past, to address these problems.
FIG. 1 is a plan view of part of a phase shifter described in U.S. Pat. No. 5,949,303. An input terminal 100 is coupled to an input feedline 101. A feedline 102 branches off from junction 103 and leads to a first output terminal 104. A second output terminal 105 is coupled to feedline 102 at junction 110 by a meander-shaped loop 106. A dielectric slab 107 partially covers feedline 102 and loop 106 and is movable along the length of the feedline 102 and over loop 106.
The leading edge 108 of the slab 107 is formed with a step-like recess 109, as shown in FIG. 2. The step-like recess 109 is dimensioned to minimize reflection of the radio wave energy propagating along the feedlines.
This arrangement suffers from several shortcomings.
Firstly, recess 109 of the moveable dielectric body 107 operates like a transformer increasing wave impedance in the direction from input terminal 100 to the output terminals. In order to have equal impedance at the input and all outputs, the device shown in U.S. Pat. No. 5,949,303 requires additional transformers between junction 110 and output terminal 104.
Secondly, all feedlines apart from 101, which is the first from input terminal 100, cross the edge of the dielectric plate twice. Therefore the reflection at two recesses can add up to double the reflection at one recess depending on the position of the dielectric plate.
Thirdly, the relative positions of the output terminals impose constraints on the layout, which may be incompatible with physical realisations of beam-forming networks for some applications.
Fourthly, it can be difficult to accurately and consistently fabricate the recess 109 in slab 107.
Fifthly, this approach is not suitable for a linear array containing an odd number of output ports.